A variety of oscillator circuits are known. Examples include the Colpitts oscillator as described in U.S. Pat. No. 1,624,537, the contents of which are hereby incorporated by reference. A Colpitts oscillator uses a “tank” circuit having an inductance with two capacitors that determine the frequency of oscillation. The feedback signal to drive the oscillator is taken from a voltage divider made of the two capacitors, connected in series.
Another example oscillator is the Clapp oscillator. The Clapp oscillator is basically a Colpitts oscillator that has an additional capacitor placed in series with the inductor.
Many applications use oscillators for tuning purposes. As such, Clapp and Colpitts oscillators are often formed as variable frequency oscillators, whose frequencies of oscillation are controlled using variable capacitors, whose capacitance(s) vary with an applied control voltage. Such oscillators are referred to as voltage controlled oscillators (VCOs).
In variable frequency, radio applications a Colpitts VCO is often preferred because of its relatively low phase noise.
Nevertheless, to tune a relatively wide frequency range, requires a capacitor having significant variability, and/or relatively large VCO control voltages. In many micro-electric and integrated circuits, required voltages and variable capacitors of such size and range are not available or impractical.
Accordingly, there is a need for an improved VCO that provides a relatively wide tuning range, with a desired precision, while using components that may be integrated and whose size may be limited for certain desired frequency ranges.